System and method for removing narrowband noise

ABSTRACT

A system and method for removing narrowband noise from an input signal in which notch filters having notch frequencies corresponding to the noise are dynamically adjusted in accordance with a detected noise spectrum. The method may be applied to telemetry systems for implantable medical devices such as cardiac pacemakers to result in improved noise immunity.

FIELD OF THE INVENTION

[0001] This invention pertains to methods and systems for removing noisefrom signals. The invention finds particular application to telemetrysystems used in implantable medical devices such as cardiac pacemakersand implantable cardioverter/defibrillators.

BACKGROUND

[0002] Implantable medical devices, including cardiac rhythm managementdevices such as pacemakers and implantable cardioverter/defibrillators,typically have the capability to communicate data with a device calledan external programmer via a radio-frequency telemetry link. One use ofsuch an external programmer is to program the operating parameters of animplanted medical device. For example, the pacing mode and otheroperating characteristics of a pacemaker are typically modified afterimplantation in this manner. Modern implantable devices also include thecapability for bidirectional communication so that information can betransmitted to the programmer from the implanted device. Among the datawhich may typically be telemetered from an implantable device arevarious operating parameters and physiological data, the latter eithercollected in real-time or stored from previous monitoring operations.

[0003] Noise refers to any unwanted signal that interferes with thetransmission and processing of data signals in a communications system.Such noise may arise from sources either internal or external to thesystem. Because of limited energy storage capability, implantablemedical devices must necessarily transmit their data with a low signalenergy, making the transmissions very susceptible to interference fromnoise. This means that an external programmer can only be satisfactorilyused to receive data in relatively noise-free environments. Because ofthe widespread nature of electromagnetic noise sources, such aconstraint may not only be inconvenient to the patient and clinician,but could also be hazardous in an emergency situation. Both broadbandand narrowband noise sources contribute to the problem, with modem CRTmonitors being a particularly common source of narrowband noise.

SUMMARY OF THE INVENTION

[0004] The present invention relates to a system and method for removingnarrowband noise from a received signal. In a particular embodiment,after digitizing the received signal, narrowband noise is removed fromthe input signal samples with a series of notch filters having centernotch frequencies generated adaptively so that the notch frequenciesmatch the frequency peaks of a detected noise spectrum. The noisespectrum is detected by first computing a power spectrum of the inputsignal and then subtracting from it a template spectrum corresponding toan expected input signal without noise. A template spectrum is computedfrom a representative input signal generated by receiving a transmittedsignal under noise-free conditions so that when it is subtracted fromthe input signal spectrum, the result approximates the power spectrum ofthe narrowband noise alone. In order to produce a detected noisespectrum that most closely approximates the true noise spectrum, thetemplate spectrum is scaled by a factor that reduces the total power inthe detected noise spectrum to a minimal value. The frequency peaks inthe detected noise spectrum are then identified and used to synthesizefilters with corresponding notch frequencies to remove the noise fromthe input signal.

[0005] The narrowband noise removal method may be employed in a systemand method for receiving telemetry data from an implantable medicaldevice to result in an improved capability for operating in noisyenvironments. In an exemplary system, the transmitted signal from theimplantable device is a radiofrequency carrier waveform modulated withdigitally encoded data in the form of transmit pulses. Further noiseimmunity may be provided to the system by matched filtering of the inputsignal samples and adaptive pulse detection.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a diagram of a telemetry system for an externalprogrammer.

[0007]FIG. 2 is a diagram of the receiver portion of the telemetrysystem.

[0008]FIG. 3 is a diagram of the signal processing functions performedby the receiver.

DETAILED DESCRIPTION

[0009] Narrowband noise, such as that generated by certain electronicdevices, is bandwidth-limited noise having a power spectrum withcharacteristic frequency peaks. Thus, a series of notch or bandstopfilters with notch frequencies that correspond to those characteristicfrequency peaks will remove the narrowband noise from an input signal inreal time. Because the power spectrum of narrowband noise found in theenvironment is not constant, however, successful removal of such noiserequires that the notch frequencies adapt to a changing noise spectrum.In accordance with the invention, a power spectrum corresponding tonoise present within an input signal is detected by subtracting atemplate spectrum from the power spectrum of the input signal. Thedetected noise spectrum is then used to synthesize the notch filtersthat remove the noise from the input signal. By continuously orperiodically detecting a noise spectrum from the input signal, the notchfilters can be resynthesized with updated notch frequencies in nearreal-time to adaptively remove noise from the input signal in responseto a changing noise spectrum.

[0010] The present invention can be applied to a telemetry datareceiving system for an external programmer to result in improvedperformance in the presence of noise. Telemetry systems for implantablemedical devices utilize radio-frequency energy to enable bidirectionalcommunication between the implantable device and an external programmer.An exemplary telemetry system for an external programmer and a cardiacpacemaker is described in U.S. Pat. No. 4,562,841, issued to Brockway etal. and assigned to Cardiac Pacemakers, Inc., the disclosure of which ishereby incorporated by reference. A radio-frequency carrier is modulatedwith digital information, typically by amplitude shift keying where thepresence or absence of pulses in the signal constitute binary symbols orbits. The external programmer transmits and receives the radio signalwith an antenna incorporated into a wand which can be positioned inproximity to the implanted device. The implantable device transmits andreceives the radio signal by means of an antenna, such as may be formedby a wire coil wrapped around the periphery of the inside of the devicecasing. As aforesaid, the limited energy storage capability of a typicalcardiac rhythm management device necessitates that the signalstransmitted from the implantable device be of low energy, thusdecreasing the signal-to-noise ratio of the signal received by theexternal programmer.

[0011] In a particular implementation of a telemetry system, datagenerated by the implantable device is transmitted in the form of acarrier signal modulated with transmit pulses representing the encodeddata. The received signal is digitized into input signal samples, andnoise is removed from the samples by two filtering operationsimplemented in the digital signal processor, one for narrowband noiseand the other for broadband noise. A series of infinite impulse response(IIR) notch filters is used to remove narrowband noise from thetransmitted signal with the filter coefficients dynamically generated inaccordance with a detected narrowband noise spectrum. (Other embodimentsmay utilize FIR or analog filters to remove the narrowband noise.) Afinite impulse response (FIR) matched filter then correlates the inputsignal with a signal corresponding to a transmit pulse in order toremove broadband noise. (In other embodiments, matched filtering can beperformed with an IIR or analog filter.) Further noise immunity isprovided by dynamically adjusting the threshold at which pulses aredetected from the output of the matched filter in accordance withmeasured noise and signal peaks.

[0012]FIG. 1 is a block diagram of the telemetry system of an externalprogrammer. The telemetry processor 50 supervises the operation of thetelemetry system, processes the data generated by it, and handlesprotocol functions such as timing, serial-parallel conversions, andcyclic redundancy code (CRC) checks. The telemetry processor 50communicates with the main host processor 30 of the programmer over ahost bus 40. The telemetry digital signal processor (DSP) 100 performsmost of the basic processing functions for the telemetry system. Itcontrols the transmitter, monitors the ambient noise level, and mayperform some protocol functions. As described below, the DSP 100 is alsoresponsible for matched filtering of the input samples, creating optimalnotch filters for removing narrowband noise in the local noiseenvironment, and dynamically adjusting the threshold signal level atwhich pulses are detected. A configuration and status channel 214between the DSP and telemetry processor allows the telemetry processorto configure the telemetry system for a particular implantable device,monitor the received signal strength, set automatic or fixed transmitterpolarities, read the wand status (i.e., presence and type), and updatethe DSP firmware.

[0013] The transmitter portion of the telemetry system is controlled bythe DSP and includes a transmitter power supply 212, a power driver 210,and a transmit filter 208. The transmitter power supply providesvoltages that are compatible with the telemetry wand antenna andprovides adjustability of the transmit power by the DSP. The powerdriver is controlled by the DSP and generates square waves that minimizeinterference with surface ECG and pace detection. The transmitter filterremoves high-frequency components of the power driver's waveform thatmay cause radiative interference with other devices. A wand antenna 205is used for both transmitting and receiving signals. The wand styledetector 206 senses both the presence of a wand and the wand type bymeasuring the resistance of a wand identification resistor. This allowsthe telemetry system to adjust the transmitter and receiver as necessaryfor particular types of wand antennas. The detector also causes thesystem to disable the transmitter if the wand is disconnected. Theanalog portion of the receiving circuitry includes a filter/amplifier204 that amplifies signals received by the wand as necessary and appliesthe low-pass anti-aliasing filtering to the signal prior toanalog-to-digital conversion by A/D converter 202. The DSP controls thefilter/amplifier's overall gain to adjust for the responses of differenttypes of wands.

[0014]FIG. 2 is a block diagram of the components making up the receiverportion of the telemetry system. The wand antenna 205 transduces achanging magnetic field intensity to a voltage which is the input signalto the analog receiver circuitry. The filter/amplifier 204 includes gaincircuitry 204 a that is distributed throughout the receiver and iscontrollable by the DSP, and a filter 204 b that provides ananti-aliasing function with its poles distributed throughout the analogreceiver circuitry. In an exemplary embodiment, a 100 KHz carrier signalis ASK modulated with a pulse train sub-carrier encoded with digitaldata, and the transmit pulses occur at a typical rate of 4 KHz with apulse width between 20 and 100 microseconds, resulting in a bandwidth ofthe modulated carrier of approximately 10 to 150 KHz. In order todigitally demodulate the carrier waveform, the analog-to-digitalconverter must then sample the received signal at a rate at least equalto the Nyquist frequency of 300 KHz. In order to provide goodcorrelation peaks in the matched filter used to detect transmit pulsesand to simplify the DSP code, the analog-to-digital converter shouldpreferably sample at a somewhat higher rate (e.g., approximately 350-400KHz). The resolution of the A/D converter should also be at least 10bits in order to provide dynamic range without an automatic gain controlcircuit. In an exemplary embodiment, a 150-kHz, seventh-orderButterworth filter provides the anti-aliasing function prior tosampling, and a 10-bit analog-to-digital converter (ADC) 202 withintegrated sample and hold generates the input samples. A feedbackmechanism within the analog receiver regulates a voltage bias to thereceiver input which tends to remove any low frequency components fromthe input signal. The output of the ADC is a synchronous serial datastream which is sent to the DSP, and the DSP controls the sample rate ofthe ADC.

[0015]FIG. 3 is a block diagram of the functions performed by thedigital signal processor 100. When the DSP receives a sample from theADC, an interrupt is generated. The receiver interrupt handler 110executed by the DSP processes the samples in the time domain with notchfilters 112 and a matched filter 113, digitizes the presence or absenceof transmit pulses via pulse detector 114, and then sends this digitaldata signal TEL_RX to the telemetry microprocessor 50. The receiverinterrupt handler also fills a 512 element noise buffer 115 withconsecutive raw input samples. When this buffer is filled, the filtergenerator task 120 processes the buffered data to generate new notchfilter coefficients. The receiver interrupt handler then uses thesecoefficients to adaptively filter out narrowband noise. A noise peakdetector 116 and a signal peak detector 117 detect and save peak signalvalues and peak noise values, respectively. These peak values areperiodically processed by the threshold adjustment task 130 in order toadaptively set the threshold that the pulse detector 114 uses todigitize the serial stream.

[0016] An integer conversion routine 111 initially subtracts an offsetfrom the input sample to convert the sample from an unsigned integer toa signed integer and remove any bias added by the analog receiver. Thesample is then processed through a six biquad IIR filter 112. Eachbiquad is either a notch filter or a simple pass-through function sothat zero to six notch filters may be active at any time. The purpose ofthe notch filters is to remove narrowband noise from the input signalsamples. Since the presence and frequency of this noise depends on theambient environment, the notch filter coefficients are adaptivelygenerated in response to detected narrow band noise. The filtergenerator task 120 does this by processing the raw input data in thebuffer 115 and periodically updating the IIR filter coefficients.

[0017] In order to obtain an optimum frequency response characteristic,the notch filters in this implementation are recursive filters (i.e.,infinite impulse response) with adaptively generated filter coefficientsso that the notch frequencies match the frequency peaks of a detectednoise spectrum. The noise spectrum is detected by first computing apower spectrum of the input signal. The receiver interrupt handler 110fills a 512 element buffer 115 with consecutive raw input samples. Whenthis buffer is full, this task then scales the buffer values up to limitround-off noise in later calculations at block 121, applies a windowingfunction such as a Hamming window to the data to limit spectralspreading at block 122, and then discrete Fourier transforms the timedomain data into frequency domain data via a Fast Fourier Transform(FFT) algorithm at block 123. The FFT output is then transformed into apower spectrum by taking the norm of the FFT output at block 124. Thereceiver interrupt handler then fills the buffer again, and the mean ofeight consecutive power spectra is taken by block 125. This averagepower spectrum is then processed by noise spectrum detector 126 in orderto detect narrow band noise peaks by subtracting from it a templatespectrum corresponding to an expected input signal without noise. Atemplate spectrum is pre-computed from a representative input signalgenerated under noise-free conditions so that when it is subtracted fromthe input signal spectrum, the result approximates the power spectrum ofthe narrowband noise alone. In order to produce a detected noisespectrum that most closely approximates the true noise spectrum, thetemplate spectrum is scaled by a factor that reduces the total power inthe detected noise spectrum to a minimal value. The frequency peaks inthe detected noise spectrum are then identified and used to synthesizefilters with corresponding notch frequencies to remove the noise fromthe input signal. The notch filters are synthesized with well-knownfilter synthesis algorithms by filter synthesizer 127.

[0018] An exemplary implementation of the filter coefficient updatingmethod just described is as follows. Let P^(i) be the power spectrum ofthe input signal and P^(e) be the template spectrum corresponding to thenoise-free signal. The detected noise spectrum P^(n) is then computedas:

P _(n) =P _(i) −R*P _(e)

[0019] where R is a scaling factor chosen to minimize P_(n). As a firstapproximation, R is set to a ratio of P_(i) to P_(e) computed bydividing P_(i) by P_(e) for each frequency bin, totaling up theseratios, and taking the average ratio. A successive approximationapproach is then used to find the optimal scaling factor. First, valuesfor R are found that produce a positive P_(n) and a negative P_(n),referred to as R+ and R−, respectively. Since between these two valuesis the value of R that minimizes P_(n), new values for R are computed asthe average of R+ and R−. As each new R value is tried in the aboveequation, it replaces the previous value of R+ or R− according towhether P_(n) is made negative or positive, respectively. The procedureis iterated until the optimal value of R is found to result in the noisespectrum P_(n). Spectral threshold values for setting the notch filtercoefficients are determined by computing the mean and standard deviationof the spectrum P_(n). In a preferred embodiment, the spectralthresholds are then set at three standard deviations above the mean.These spectral thresholds then constitute the frequency peaks used toset the notch frequencies of the notch filters 112.

[0020] Referring to block 110 of FIG. 3, the output of the notch filterstage is input to the FIR matched filter 113. The coefficients of theFIR filter 113 are designed to correlate the filtered input signalsamples with samples that would be expected from a transmit pulsegenerated by the implantable device. This type of filter is veryeffective in discriminating transmit pulses from background noise andincreases the range of the telemetry system. The FIR coefficients arederived by capturing a strong, noise-free transmission signal from theimplantable device immediately after the samples are converted to signedintegers in the receiver interrupt handler. The captured data is thenmanipulated so that the signal samples are reversed in their order, thusflipping them in time, and each sample is amplitude offset so theaverage of the samples is near zero in order to eliminate any DCcomponent from the coefficients. The samples are then normalized so thatthey are fractions, with the maximum sample amplitude equal to 1.0.These fractions are then scaled so the results are in the range of−32768 to 32767 and then copied into the appropriate FIR coefficienttable. With these FIR filter coefficients, the matched filter 113performs a convolution between the input signal samples and samplescorresponding to a time-reversed version of the transmit pulse expectedto be generated by the implantable device, which is equivalent toperforming a cross-correlation between the input signal and a transmitpulse. The output of the matched filter 113 is then compared to athreshold value (td_threshold) by the pulse detector 114. The TEL RXsignal is set high if the filtered value is above td_threshold,otherwise TEL RX is set low.

[0021] The FIR filter output noise and signal peak values are calculatedby peak detectors 116 and 117 which are then saved for processing by thethreshold adjustment task 130. Signal values are discriminated fromnoise values based on the timing of the sampled data relative to thelast transmit pulse. If a sample occurs at a time when the telemetryprotocol does not allow a transmit pulse from the implantable device,then the sample is assumed to be noise, otherwise it is assumed to be asignal. These peak values are periodically processed by the thresholdadjustment task 130 in order to adaptively set the value of td_thresholdthat the pulse detector 114 uses to digitize the serial stream.

[0022] The threshold adjustment task 130 uses the peak noise and signalvalues calculated by the receiver interrupt handler to update the valueof td_threshold. The threshold is dynamic so that best spurious noiserejection is accomplished in noisy environments and maximum sensitivityis accomplished in noise-free environments. A local variable,min_threshold, is maintained. This variable is used to set a lower limitto the value of td_threshold. It can rapidly increase in value, but canonly slowly decrease in value. If the noise peak is greater thanmin_threshold as determined at step 131, then min_threshold is assignedthe noise peak value at step 132. This is done so that spurious noisewhich makes its way through the digital filters can be rapidly respondedto. Note that Gaussian noise will statistically attain very large valueson rare occasions, so min_threshold will track to the Gaussian noisepeaks, not the average level. If the noise peak is not greater thanmin_threshold, then the peak value is averaged into min_threshold usinga weighted moving average at step 133. The peak value is lightlyweighted, so that the decay rate of min_threshold is relatively slow.This slow averaging is done because min_threshold is designed to guardagainst spurious noise conditions (i.e., noise absent over a shortinterval does not necessarily mean the noise has gone away).

[0023] The signal peak is then compared to min _threshold at step 134.If it is below min _threshold, then a transmitted signal is assumed tobe absent, and td_threshold simply remains at its current value. If thesignal peak is above min _threshold, then the signal peak value isaveraged into a local store using a weighted moving average at step 135.The value of td_threshold is then set to half the value of this localstore. Thus, the value of td_threshold tends to be one half the value ofthe transmit pulse peak value. The signal peak is weighted relativelyheavily, so that td_threshold can react to variations in telemetry rangethat normally occur as the operator manipulates the wand. Note that ifthe calculated value of td_threshold is below min_threshold, thentd_threshold is clamped to the value of min_threshold. Also, although itis desirable to rapidly change td_threshold in order to react to rangevariations, it is not desirable for it to change too quickly. Since thematched filtered transmit pulses have a finite slope, the threshold atwhich a signal is detected will affect the time at which the digitizedoutput changes state. Since the time domain FIR filter output tends tohave a shape similar to a Sinc function, rapid variations intd_threshold could detect only the main lobe for some data bits, andleading side lobes for other data bits. Certain synchronizationprotocols are particularly sensitive to this problem, since they use analignment bit to establish the data window timing for the subsequentdata bits. The threshold should therefore preferably be stable from whenthe alignment bit is detected to when the last data bit is detected.Thus, the averaging weight of the peak signal is preferably chosen toachieve the best compromise between responsiveness to range variationand threshold stability during receipt of transmitted word.

[0024] In the embodiments of the invention described above, the receivedsignal was digitized and processed in the digital domain to derive thetransmit pulses. In other embodiments, the received signal could beprocessed in the analog domain to remove narrowband and narrowbandnoise, correlate the signal with a transmit pulse by matched filtering,and detect transmit pulses with an adaptive threshold.

[0025] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A system for removing narrowband noise from areceived signal comprising: one or more notch filters for removing thenarrowband noise from the signal with the filters having notchfrequencies corresponding to frequency components of the narrowbandnoise; means for detecting a noise spectrum of the received signal; and,means for updating filter coefficients of the notch filters so that thenotch frequencies correspond to the detected noise spectrum.
 2. Thesystem of claim 1 further comprising an analog-to-digital converter fordigitizing the received signal into input signal samples, and whereinthe notch filters are digital filters.
 3. The system of claim 2 whereinthe notch filters are digital infinite impulse response filters andfurther comprising a filter generator for synthesizing the notch filterswith notch frequencies corresponding to the detected noise spectrum. 4.The system of claim 2 wherein the noise spectrum detecting means detectsa noise spectrum by discrete Fourier transforming a set of input samplesto produce a power spectrum of the input signal and subtracting atemplate spectrum therefrom, the template spectrum corresponding to arepresentative input signal without noise.
 5. The system of claim 4wherein the template spectrum is scaled by a factor that reduces thetotal power in the detected noise spectrum to a minimal value.
 6. Thesystem of claim 4 wherein the input signal power spectrum is produced byperforming a discrete Fourier transform on a set of input samples toproduce a set of corresponding Fourier coefficients and normalizingthose coefficients.
 7. The system of claim 4 wherein the input signalpower spectrum is produced by summing the power spectra of a series ofinput sample sets.
 8. The system of claim 4 wherein the filter generatorsets the notch frequencies of the notch filters to match those spectralcomponents of the noise spectrum that exceed a specified spectralthreshold value.
 9. The system of claim 8 wherein the specified spectralthreshold value is calculated as a specified number of standarddeviations from the mean of the noise spectrum.
 10. The system of claim4 wherein the filter coefficients of the notch filters are periodicallyupdated by the filter generator.
 11. The system of claim 10 wherein thefilter coefficients of the notch filters are updated based upon inputsamples continuously input into a buffer.
 12. A method for removingnarrowband noise from a received signal comprising: filtering thereceived signal with one or more notch filters, the notch filters havingnotch frequencies corresponding to frequency components of thenarrowband noise; and, updating filter coefficients of the notch filtersso that the notch frequencies correspond to a detected noise spectrum ofthe transmitted signal.
 13. The method of claim 12 wherein the receivedsignal is digitized into input signal samples.
 14. The method of claim13 further comprising detecting a noise spectrum by discrete Fouriertransforming a set of input signal samples to produce a power spectrumof the input signal and subtracting a template spectrum therefrom, thetemplate spectrum corresponding to a representative received signalwithout noise.
 15. The method of claim 14 further comprising scaling thetemplate spectrum by a factor that reduces the total power in thedetected noise spectrum to a minimal value.
 16. The method of claim 15further comprising performing a discrete Fourier transform on a set ofinput samples to produce a set of corresponding Fourier coefficients andnormalizing those coefficients to result in the input signal powerspectrum.
 17. The method of claim 14 further comprising summing thepower spectra of a series of input sample sets to result in the inputsignal power spectrum.
 18. The method of claim 14 further comprisingsetting the notch frequencies of the notch filters to match thosespectral components of the noise spectrum that exceed a specifiedspectral threshold value.
 19. The method of claim 18 further comprisingcalculating the specified spectral threshold value as a specified numberof standard deviations from the mean of the noise spectrum.
 20. Themethod of claim 12 further comprising periodically updating the filtercoefficients of the notch filters.
 21. The method of claim 20 furthercomprising updating the filter coefficients of the notch filters basedupon input samples continuously input into a buffer.
 22. The method ofclaim 13 wherein the notch filters are digital infinite impulse responsefilters synthesized with notch frequencies corresponding to the detectednoise spectrum.
 23. A system for receiving data in the form of transmitpulses from an implantable medical device, comprising: an antenna forreceiving a transmitted signal from an implantable device; ananalog-to-digital converter for digitizing the received signal intoinput signal samples; one or more notch filters for removing narrowbandnoise from the received signal with the filters having notch frequenciescorresponding to frequency components of the narrowband noise; means fordetecting a noise spectrum of the received signal; and, means forupdating filter coefficients of the notch filters so that the notchfrequencies correspond to the detected noise spectrum.
 24. The system ofclaim 23 wherein the notch filters are digital infinite impulse responsefilters and further comprising a filter generator for synthesizing thenotch filters with notch frequencies corresponding to the detected noisespectrum.
 25. The system of claim 24 wherein the noise spectrumdetecting means detects a noise spectrum by discrete Fouriertransforming a set of input samples to produce a power spectrum of theinput signal and subtracting a template spectrum therefrom, the templatespectrum corresponding to a representative input signal without noise.26. The system of claim 25 further comprising: a matched filter withfilter coefficients corresponding to a transmit pulse; and, a pulsedetector for detecting transmit pulses within the input signal samplesby comparing output values of the matched filter with a pulse thresholdvalue wherein the pulse threshold value is adjusted in accordance withmeasured peak values of noise and the transmitted signal.
 27. The systemof claim 26 wherein noise peak values are measured at a time when theimplantable device is known not to be transmitting in accordance with atelemetry protocol.
 28. The system of claim 27 wherein the pulsethreshold value is computed as a fraction of a weighted moving averageof measured signal peak values that exceed a specified minimum pulsethreshold value.
 29. The system of claim 28 wherein the minimum pulsethreshold value is adjusted in accordance with measured noise peakvalues.
 30. The system of claim 29 wherein the minimum pulse thresholdis adjusted by comparing a current minimum pulse threshold value with ameasured noise peak value, setting the minimum pulse threshold valueequal to the measured noise peak value if the measured noise peak valueis greater than the current minimum pulse threshold value, and settingthe minimum pulse threshold value equal to a weighted moving average ofmeasured noise peak values if the measured noise peak value is less thanthe current minimum pulse threshold value.